1. Field of the Invention
This invention relates generally to a contact probe employed for a semiconductor manufacturing in-line-monitoring process and for semiconductor device measurement More particularly, this invention relates to a low-melting contact pads for establishing instant electrical connections suitable for direct thin-film measurement without requiring preparation and pending time or using special fabrication equipment while achieving higher measurement accuracy.
2. Description of the Related Art
In order to control the ion contamination in a semiconductor-manufacturing environment, a time-consuming and difficult task of in-line measurement of mobile ions in a semiconductor wafer must be performed to qualify and continuously monitor the fabrication equipment This process is required due to the fact that the performance characteristics of an integrated circuit (IC) device is often adversely affected by mobile ion contamination introduced during the fabrication processes. Contamination of alkali metal ions, e.g., sodium and potassium ions, can affect the threshold voltage of a MOSFET device and degrade the gate oxide rupture voltage and channel mobility. Mobile ion contamination may also cause device reliability problem due to the fact that under an electrical bias, the contaminants move slowly at room temperature. But when the temperature is elevated, the mobile ions are moved at very high speed under the influence of the electrical field. Such a moving speed variation at different temperature of the contaminant ions often causes undesirable spurious performance characteristics and leads to reliability problems.
Since the undesirable mobile ions can be generated from a wide variety of sources such as human body, furnace, photo-resist, chemical etchants, etc., the fabrication equipment must first be qualified prior to wafer processing steps are carried out initially. The mobile ion density must also be closely monitored to control the contamination level during the entire fabrication process. Speed and accuracy of mobile ion measuring tools can have significant impact on the productivity and quality of the device manufacture. For those of ordinary skill in the art, there are several in-line measurements and probing techniques being applied for measuring and monitoring the mobile ion contamination.
A capacitance voltage (C-V) in-line measurement system is often employed in the industry. The mobile-ion contamination level is determined by measuring the parallel voltage shift (.DELTA.V) between the C-V curve before and after the bias temperature stress. The technique is based on the fact that the mobile ion density (Q.sub.m) drifting from the metal-SiO.sub.2 interface and the bulk oxide to SiO.sub.2 --Si interface at a given positive gate bias is proportional to the voltage shift (.DELTA.V), i.e., Qm=C.sub.ox .DELTA.V. This measurement technique is very time-consuming since the C-V measurements before and after the stress are to be performed at room temperature while the bias stress measurement is to be carried out at an elevated temperature above 150.degree. C. At least thirty minutes are required to complete this test due to the time wasted in waiting for the temperature to cool down to the room temperature when a conventional C-V measurement is carried out.
Another method for mobile ion measurement is carried out by the use of a triangular voltage sweep (TVS) technique. A triangular voltage ramp is applied to the gate and an ionic displacement current is measured at an elevated temperature. The contamination level is calculated from gate current due to the mobile ion drift from the oxide bulk to an interface of gate oxide and silicon. The mobile ion density, i.e., Q.sub.m, drifting at a given temperature is proportional to the area under the peak in the gate current Ig caused by the ionic motion, i.e., Q.sub.m =.SIGMA.I.sub.g .DELTA.V.sub.g where V.sub.g is the gate bias. The TVS method, even that it requires more complicate analyses, can achieve higher speed of operation and is more accurate because unlike the C-V technique which employs two C-V curves to determine the mobile ion density while the TVS method only needs one I-V curve. Furthermore, the TVS measured sample can be maintained at an elevated temperature while taking data without requiring cooling off to room temperature. Additionally, when a TVS method is applied, the mobile ion density can be accurately measured without be affected by the interface trap normally formed at SiO.sub.2 --Si interface during an oxidation process. In contrast to C-V measurement, the interface trap density varies significantly due to heating, which in turn leads to a non-parallel voltage shift thus causing measurement inaccuracies. However, since the measurement technique involves an integration computation as explained above, an on-line computer is required with a computation-software and a data-entry device for carrying out the task of ion density measurements. Because different alkali ions have different drift rates, the peak gate current occurs at different gate bias, therefore different mobile ion species such as sodium and potassium, when measured by using the TVS method, can be separated at a given temperature. Both of the above two techniques require that the measurement to be performed at higher temperatures. Furthermore, there are still no contact or contactiess probes applying the above two techniques, which can measure the mobile ions directly.
FIG. 1A shows a mercury probe that is another contact probe commonly used for in-line process monitor in the industry. A mercury probe has been widely used for epitaxial (EPI) resistivity and oxide fixed charge measurement Due to the high vapor pressure at higher temperature and the concerns that the mercury vapor may induce health hazards, a mercury probe is not suitable for high-temperature mobile-ion measurement.
FIG. 1B shows a metal pin probe which is commonly used by first developing aluminum dots on wafers prior to applying a metal pin probe is applied for testing. Referring to FIG. 1C for the configuration of forming the aluminum dots with a metal deposition process on a silicon oxide layer on a silicon substrate for establishing an electric connection between the metal pin probe and the substrate. This requirement of metal dot development not only causes the equipment down time during the formation of these dots that takes at least eight hours. The metal deposition system for dot development may introduce unexpected mobile ions and cause physical damages to the wafer, which leads to inaccurate ion level measurements. Another limitation of the metal pin probe is the pin-slip from the dots caused by the thermal expansion of the wafer and the thermal chuck when there is a high temperature induced stress. The pin slip may often cause the inaccuracy in ion contamination measurement.
FIG. 1D shows a four-point probe including four sharp metal pins which is commonly employed for in-line sheet resistance measurement Such probe does not provide adequate measurement accuracy required espedally for shallow junction where the depth is dose to the probe damage range, e.g., 0.1 to 0.5 microns. For a shallow junction that is formed by rapid thermal annealing, the four-point probe does not provide sufficient accuracy for an in-line operation to monitor the ionimplant dosage.
Besides the above measuring techniques, a contactiess probe is not useful for the measurement of mobile ion contamination since the movement of mobile ions requires the presence of an electrical field. For the above reasons, there is still a need in the art of in-line process monitoring, e.g., mobile-ion measurement, to provide new and improved measurement techniques and devices to overcome the limitations and difficulties discussed above.